As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. Two main approaches currently being investigated are thermally assisted magnetic recording (TAMR) and microwave assisted magnetic recording (MAMR). The latter is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). MAMR uses a spin torque device to generate a high frequency field that reduces the coercive field of a medium bit thereby allowing the bit to be switched with a lower main pole field.
Spin transfer torque devices (also known as STO devices) are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high. STO devices are also referred to as one of the spintronic devices and have ferromagnetic (FM) layers that may have a perpendicular magnetic anisotropy (PMA) component where magnetization is aligned substantially perpendicular to the plane of the FM layer. However, unlike Magnetoresistive Random Access Memory (MRAM) where PMA is necessary to keep magnetization perpendicular to plane in a free layer and reference layer, for example, STO in MAMR and related applications has a sufficiently strong write gap field to align magnetization in magnetic layers without requiring inherent PMA in the layers.
MAMR typically operates with the application of a bias current from the main pole across the STO device to a trailing shield, or vice versa, in order to apply spin torque on an oscillation layer (OL) so that the OL's oscillation generates a high frequency RF field. The RF field induces a precessional state and lower coercivity in a magnetic bit to be written in a magnetic medium. Simultaneously, a write field from the main pole is applied from an air bearing surface (ABS) to the magnetic medium, and lower field strength is needed to write the bit because of the RF field assist. In spin-torque-assisted FGL reversal schemes, FGL magnetization flips to an opposite direction when the applied current is sufficiently large enough thereby increasing the write gap reluctance, which causes a greater write field output. Both MAMR and magnetic reversal typically require a relatively high current density (>108 A/cm2) to in order to apply a useful spin torque effect for generating a RF field or for FGL flipping. The oscillation cone angle in the FGL becomes smaller with increasing current density to substantially shrink the MAMR effect. Accordingly, no STO design exists that enables a substantial spin-torque-induced FGL reversal effect while simultaneously providing a significant MAMR effect. Thus, an improved STO device is needed with a structure that allows both of the spin-torque-induced FGL reversal assist effect and MAMR effect for improved write performance over a structure where only one of spin torque assist and MAMR is applied and the other is essentially ineffective.